1. Field of the Invention
The invention is generally related to tool positioning assemblies and particularly to a tool manipulator assembly for use inside a steam generator.
2. General Background
Nuclear power plant steam generators are water to water heat exchangers. Heat is transferred from the pressurized primary. coolant to the secondary coolant. The secondary coolant is transformed into steam to drive the turbine generators. Primary coolant enters the steam generator from the reactor vessel via the inlet or hot leg plenum. As the fluid passes through the plurality (3000-15,000) of heat exchanger tubes, heat is transferred across the tube surface to the secondary fluid. The primary coolant then exits through the steam generator outlet plenum. The ends of the heat exchanger tubes pass through and are captured in a thick plate called a tubesheet. The tubes and tube sheet form the boundary between the primary and secondary coolant. The secondary coolant flows through the cylindrical chamber which encircles the heat exchanger tube bundle.
The codes and standars which regulate the nuclear industry require heat exchanger tubes inside of a steam generator be inspected on a regular basis. Further, individual tubes must be repaired or taken out of service as indicated by the results of the required inspections. Access to the tubes is gained via flanged openings in both the inlet and outlet plenums called manways. The surfaces of the plenums are contaminated with radioactive material. Radiation dose rates and airborne coontamination level inside the plenum dictate restrictive controls on a worker entering the plenums. These restrictions include protective clothing, respiratory protection, and time limits. These restricitons coupled with an industry wide move to reduce occupational radiation exposure have forced steam generator service vendors to use remotely operated manipulators to perform steam generator repair and inspection services.
The primary feature of an advanced remote steam generator manipulator is to minimize personnel radiation exposure. This is accomplished by designing the manipulator for remote installation and removal and by having the ability to change tool heads without personnel breaking the manway plane. Secondary features include precise positioning and tool alignment at each tube location, aequate strength and stiffness to support various repair tasks, rapid positioning speed, high reliability, and fail-safe design features to minimmize the potential for damage to the steam generator.
Several manipulator designs are known. Hydraulically operated designs provide strength-to-weight advantages. However, concerns over leakage of hydraulic fluid into the steam generator have minimized use of such systems. Electric or pneumatic systems used may be generally grouped into fingerwalker, mast mount, or manway mount types. Fingerwalker types are designed to hang from and walk along the tubesheet. This design suffers from slow travel speeds. It also can not accommodate large numbers of plugged tubes which may prevent one or more fingers from engaging in the tubesheet. There is also the potential for the device to fall from the tubesheet. Mast mount tubes are designed to operate from a mast bridging between the tubesheet and the bowl bottom of the generator. The mast prevents access to certain tubes and must be moved to allow access to those tubes. Manipulator installation and tool changes can also be difficult and require partial entry by personnel into the generator. This increases radiation exposure. Problems associated with known manway mount manipulators include poor stiffness, limited lift capacity, blockage of the manway, limited vertical travel, and the need for manned entry into the steam generator for toolhead changes.
It can be seen that a need exists for a tool manipulator for use inside a steam generator that solves the above problems.